I have just completed an essay on Paul's use of the phrase 'ambassadors for Christ' in 2 Cor. 5.20. Here is an excerpt from the conclusion:
But perhaps the greatest challenge that Paul’s description of the work of ‘ambassadors for Christ’ leaves with us is the idea that the definition and practice of ministry (whether that belonging to all Christian disciples or appointed Christian ministers) must always be judged against the criteria of the gospel. The notion that Paul’s language in 2 Corinthians 5 evokes and invites participation in the narrative of the saving work of the Servant of YHWH reminds us that decisions about what ministry is or might be depend to a large extent on our view of the story that the church is a part of. If we struggle to understand what ministry is, then perhaps a part of the reason is that we do not see the gospel clearly enough. In the current climate, the answer to the question ‘what is ministry?’ or more specifically ‘what is a minister?’ will often take the form of some kind of list of skills, activities, personal characteristics and above all ‘competencies’ that make ministry recognisable, visible and accreditable as such. Perhaps Paul’s rendition of the Isaianic drama in which God overcomes the enmity between Godself, God’s people and the nations, through the suffering and vindication of a Servant, whose story is in turn narrated by ‘servants of the servant’ (a story that Paul summarizes as ‘God was in Christ reconciling the world to himself … and entrusting the message of reconciliation to us’) serves to remind us that that the shape of effective ministry is to be determined by the dramatic and disruptive event of God’s advent in Christ. Ministry is located in the midst of this story, takes its cue from it, and points others to it. To be a ‘minister’, to be an ambassador for Christ, is to live in, look for and direct others to the new creation that is found in Christ.
 This formulation owes much to Alasdair MacIntyre’s account of the nature of ethical reflection: ‘I can only answer the question “What am I to do?” if I can answer the prior question “Of what story or stories do I find myself a part?”.’Alasdair MacIntyre, After Virtue: A Study in Moral Theory (2nd edn.; London: Duckworth, 1985), p.216.